Melbourne, Jan. 27 : Defending champion Novak Djokovic has quit the Australian Open, retiring from his quarter-final against Andy Roddick on Tuesday.

Djokovic, the No. 3-seed, from Serbia, was trailing the No. 7 seed, from the United States, 6-7 (5-7) 6-4 6-2 2-1 when he quit the match, after two-and-a-half hours, apparently suffering from heat stress.

The Serb has similarly quit mid-match during the latter stages of a grand slam tournament twice before - in the 2006 French Open quarterfinals and the 2007 Wimbledon semi-finals, against Rafael Nadal on both occasions.

Djokovic had given little indication of his problems to come during the early stages of the match, but he started looking towards the players'' box at the start of the third set and soon called for a medical time out to have his legs iced.

He returned to the match, but he looked lethargic and eventually talked to the doctor once more before deciding that he couldn''t continue. (ANI)